Answer:
Selective Dropout
Explanation:
According to my research on different research concerns and common issues, I can say that based on the information provided within the question Dr. Salahma needs to be concerned about Selective Dropout. Like mentioned in the question this term refers to the tendency of some individuals to drop out of a study. Which can make the results of that study completely invalid. Which is what seemed to have happened in the first few trials of the experiment mentioned in the question.
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